In the days when American cities produced things you could hold and touch, Philadelphia built sprawling factories like the Budd and Baldwin plants. Now that hospitality has become the linchpin of our urban economies, cities instead vie to build ever bigger and grander convention centers.

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This week, it will be Philadelphia's turn, when it opens the largest exhibition hall on the East Coast, joining an elite crowd capable of hosting mega-conventions of 20,000 or more people. The culmination of a decade of effort, the expanded Convention Center is so vast, it could roof over three Rittenhouse Squares. Other cities may claim bigger boxes, but none can match the downtown location.

The hopes that have been invested in the $786 million project are as enormous as the structure. Not only is the city counting on the enlarged Convention Center to create jobs and spawn new hotels and restaurants, but it also believes the building can awaken the sleeping beauty that is North Broad Street. That last item, however, may be too much to expect from a work of architecture, never mind one that will be dark more than 170 days this year.

Like the factories that once dominated the latitudes north of City Hall, the expanded center is designed for function - the moving, seating, and feeding of large groups, quickly and efficiently. These are things it appears capable of doing very well.

As for the architecture, its only role, unfortunately, is to camouflage the center's bulk, much like the false fronts used to adorn frontier buildings. Even at that, the screening is applied to only two of the building's four main facades. Long stretches of blankness and inactivity make it difficult for this gargantuan box - covering six full city blocks - to sustain the kind of meaningful relationship with its neighbors that we expect from our urban buildings.

Produced by a team of architects under the leadership of Atlanta's TVS Design and Philadelphia's Vitetta, the supersized Convention Center is a virtual clone of the original 1993 building. It exhibits all the same strengths and limitations of that 18-year-old design, with one notable improvement: Now people will be able to find the front door.

As befitting such a potent economic engine, the enlarged Convention Center has been given a major entrance on Broad Street, one that is commensurate with its physical and symbolic presence. The 11-story-high shield of curving glass is easily the best of four main facades.

To be sure, there's a bit of airport-modern styling in the arrangement of horizontal sunshades and aluminum-sheathed columns. But the monumental wall effectively reorients the Convention Center, placing it at the doorstep of Philadelphia's downtown office district. The original entrance, an insignificant corner door next to the murky 12th Street tunnel, always seemed to lack the proper ceremony that such a prominent building deserves.

Now the Broad Street facade puffs out its glass chest, as if to assert the center's belatedly recognized importance in the city's orbit. At night, bolts of colored LED lights shoot across the horizontal fins, injecting some needed energy into both the facade and that tattered block north of City Hall.

Given the effort that went into creating such strong curbside appeal, it's hard to understand why the center installed such runty entrance doors, which look oddly out of proportion to the grandly scaled glass wall. The generic, overly shiny, off-the-shelf doors, which are said to be easy to maintain and lock, suggest a bean counter at work.

Once you wriggle through those narrow openings into the lobby's soaring atrium, the place starts to feel familiar. Escalators clad in limestone and granite break down the scale of the immense lobby, and take visitors up to the main exhibition hall. To unify the interior, the Pennsylvania Convention Center Authority had the architects repeat the same finishes, design details, and carpet pattern used in the 1993 building.

Outside on Arch Street, the architects also continued the rhythm of gabled sections developed back in the early '90s, during the waning days of postmodernism. While breaking the long facade into rowhouse-size sections was effective then in humanizing the large building, the additional run of gables between 13th and Broad makes the facade feel a bit relentless. Real rowhouse blocks are punctuated with commerce and variety. These gables seem to go on forever.

The expansion's star attraction is the Terrace Ballroom on the top level, which can accommodate 3,700 for dinner, 6,000 for a lecture. From the lobby outside the ballroom, the glass facade beautifully frames Broad Street's ravishing lineup of masonry buildings, including Frank Furness' masterpiece, the Pennsylvania Academy of the Fine Arts.

That view, incidentally, will be the addition's only cultural diversion. Unlike the 1993 building, the new portion has no public art in the lobby or hallways. That's because the state Department of General Services, which took over the construction project from the convention authority in 2007, killed the budget for paintings and sculpture, along with several other features that were intended to humanize the center's vast spaces.

DGS, the state's construction agency, was also responsible for tearing down a historic row of buildings on Broad Street, breaking a written covenant the convention authority had negotiated with preservationists.

The question now is what happens to the awkward gap where the buildings stood. The space was set aside to be used as a garden by the hotel that will someday occupy the Liberty Title tower at Broad and Arch. But until the building owner secures a hotel deal, the opening will remain a windswept desert of concrete.

Several other aspects of the project shortchange the public.

While one may quibble with the details, the architectural team, which includes Philadelphia's Kelly/Maiello and Synterra, clearly went to great effort to activate the Broad and Arch Street facades. Both offer glimpses into the public areas where conventioneers gather, and those views help make the enormous building feel alive.

The problem is that the stage inside will be bare far too often. Because the Broad Street addition is booked only 133 days between now and December, the impressive new lobby will stand empty much of the time, and the doors will be locked. Perhaps the convention business will pick up next year, but such inactivity challenges the assumption that the expansion can be a boon to North Broad Street.

Unlike the factories they replaced, convention centers don't have people flowing through their doors day in and day out. Their erratic use makes it difficult for shops and restaurants to put down roots nearby. No wonder the 1200 block of Arch Street remains seriously underdeveloped 18 years after the original Convention Center opened across the street.

The center's expansion was meant to rectify that situation by reducing the downtime between conventions. The enlarged meeting space allows the convention authority to break down one exhibit while it sets up the next. Now that the exhibit hall spans three city blocks, the center can also accommodate more of the huge, industrywide meetings by the likes the American Medical Association. But those prizes are rare.

Given the challenges of keeping the meeting hall filled, the convention authority might have done better to allocate retail-size space along its long facades to other users. At one stage of the decadelong design process, the authority set aside space at the Race Street corner so one of the city's art museums could open a satellite gallery. Instead, that key corner is wasted with the blandest of plazas, a concrete pad that is virtually unlandscaped.

The weakest part of the design, however, is surely Race Street, which has been turned into a three-block service entrance.

Always treated as the Convention Center's back door, it's worse now. Instead of helping to lure development north, it stops it dead in its tracks. My guess is that the momentum will spread west instead, toward Center City, PAFA, and the Parkway's cultural riches.

Convention centers, like stadiums, belong to that breed of architecture that is complete only when filled with people. On the days when conventioneers are oozing from every doorway, it won't matter so much that the building is a 20-acre box wrapped in a thin skin of glass and concrete.